[0001] The present International Patent Application claims priority pursuant to 35 U.S.C. § 119(e) to U.S. Provisional Application Serial No. 61/890,656, filed October 14, 2013, in the name of Jeremy Rice, entitled "A TWO-PHASE (LIQUID/ VAPOR), CLOSED LOOP HEAT TRANSFER SYSTEM WrfH A PASSIVE VAPOR DRF EN PUMP (VDP)," the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] There are several existing closed loop heat transfer technologies, including both passive and active methods. The passive methods are generally limited by heat load, transport distance or orientation, while the active methods require an external power source.

[0003] Heat pipes are a passive device relying on two-phase heat transfer (evaporation/condensation). The liquid is pumped by capillarity through a continuous wick, from the condenser, to the evaporator. This device is reversible (i.e., the condenser and evaporator can be switched). The heat load and distance is most frequently limited by the capillarity of the wick, but can also be limited by the sonic limit, nucleation limit, and entrainment limit. The heat transport in this device is nearly isothermal.

[0004] Looped heat pipes are similar to heat pipes (passive, two-phase heat transfer), where the condensate is returned to the evaporator through capillarity. The wick structure is only located in the evaporator, and the condensate is returned through tubes, which allows for further transport length. Special considerations for start-up are needed to ensure liquid is continuous from the condenser to the evaporator. The heat transport in this device is also nearly isothermal.

[0005] Thermosyphons are another passive two-phase heat transfer device. The condensate is returned to the evaporator via gravity. The operation of a thermosyphon is not- reversible, since the condenser must be higher, with respect to gravity, than the evaporator. The transport distance can be very high in these devices. The heat transfer in this device is also nearly isothermal.

[0006] Pumping a liquid through a closed heat transfer loop is one of the most common, and oldest closed loop heat transfer methods. It involves a pump, a heat input heat exchanger (air/liquid, liquid/liquid, refrigerant/liquid, or generically heat source/liquid), and a heat rejection heat exchanger. The pump needs power delivered from an external engine or motor. The heat transfer in this method relies on sensible heating, therefore, the liquid temperature increases and decreases in the heat absorption and rejection processes, respectively.

[0007] Similar to a pumped liquid loop, a pumped two-phase cooling loop, uses a pump, evaporator and a condenser. Condensate is pumped from the condenser to the evaporator. The pump is powered by a motor, converting electricity into mechanical energy. The heat transfer in this loop is nearly isothermal, when its operation is under low pressure differentials.

SUMMARY OF THE INVENTION

[0008] The invention is a thermally activated closed loop heat transfer system that alleviates the need for an external power source other than the heat being transferred, thus eliminating the external power overhead associated with motor driven pumps. The system is based on circulating a working fluid with a pump, part of which changes state from liquid to vapor and back, that is driven by an engine. All of the engine's power output is utilized by the pump. The system allows for nearly isothermal heat transport from the heat input to the heat output when pure or azeotropic working fluids are chosen, since the power required to circulate a fluid is miniscule when compared to the heat being transported.

[0009] The system has moving mechanical parts, in the form of a positive displacement engine directly powering a positive displacement pump. The system automatically allows for nearly proportional fluid flow relative to the total heat transported under a set operating condition, since the proportions of fluid passing through the engine and pump are fixed by the displacement ratio of the two devices. The need for external control to adjust the pumping speed is thereby eliminated.

[0010] Varying heat transport distance can readily be accommodated by the present invention, for instance distances of less than lm or greater than 100m can be accommodated by the same system and same power loads. Also the system can work against gravity, centrifugal forces or other body forces, giving the system flexibility in its applications. The transport distance and ability to work against body forces yields flexibility in operation when compared to passive devices that rely on capillarity or gravity to circulate fluid.

[0011] The foregoing has outlined rather broadly certain aspects of the present invention in order that the detailed description of the invention that follows may better be understood. Additional features and advantages of the present invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numerals indicate like features and wherein:

FIG. 1 is a schematic of a pumped two-phase closed loop heat transfer system in accordance with prior art;

FIG. 2 A is a schematic of a thermally activated closed loop heat transfer system in accordance with one embodiment of the present invention;

FIG. 2 B is another schematic of the thermally activated closed loop heat transfer system shown in FIG. 2A which has been simplified for use in subsequent representations;

FIG. 3 is a representation of the present invention on a temperature-entropy diagram under ideal and real conditions, with a pure or azeotropic working fluid;

FIG. 4 is a representation of the invention on a temperature-entropy diagram with a zeotropic working fluid;

FIG. 5 is an assembled view of one embodiment of the engine and pump of the present invention;

FIG. 6 is a blown apart view of one embodiment of the engine and pump assembly of the present invention, consisting of lobes and gears; and FIG. 7 A is a schematic of a system with several engine and pump assemblies of the present invention implemented in parallel and sharing a common condenser: and

FIG. 7 B is a schematic of a system with several engine and pump assemblies of the present invention implemented in parallel and sharing a common evaporator.

DETAILED DESCRIPTION OF THE INVENTION

[0013] A typical pumped two-phase closed loop heat transfer system is presented in FIG. 1. This system consists of a motor 100, a pump 104, an evaporator 101, and a condenser 102. The pump 104 which is driven by the motor 100, receives it's liquid from the condenser 102, and delivers the liquid to the evaporator 101. There are three energy exchanges into or out of this system. Electrical energy is input into the motor 100, which is converted into mechanical energy. Thermal energy is input into the evaporator 101, which vaporizes the liquid. Thermal energy is removed from the condenser 102, which condenses the vapor back to liquid.

[0014] One embodiment of the present invention is represented in a basic schematic in FIG. 2A, and includes an evaporator 101, a condenser 102, an engine 103 and a pump 104. The engine and pump are both positive displacement devices, and all power generated by the engine 103 drives the pump 104. While the engine 103 is a positive displacement type, it should be designed for an isochoric (constant volume) process since the pressure differential across the device is small. In this system there are only two energy exchanges into and out of the system. Thermal energy is input into the evaporator 101 where it vaporizes the liquid supplied by the pump 104. The fluid leaving the evaporator 101 has a higher pressure than the fluid entering the condenser 102. The access energy is recovered by the engine 103 and converted into mechanical energy. The mechanical energy is used to overcome the frictional losses associated with the fluid movement in the components and interconnecting tubes, as well as the friction associated with the pump 104 and the engine 103. The evaporator 101 may be a single evaporator or multiple evaporators in series or in parallel. The condenser 102 may also be a single condenser, multiple condensers in series or multiple condensers in parallel.

[0015] A simplified schematic of the invention is presented in FIG. 2B. This schematic is equivalent to FIG. 2A, however, since the all of the engine's 103 power is utilized to drive the pump 104, and the pump 104 requires no extra power, these components are coupled into a single component 105 in the schematic, hereby referred to as the heat transfer engine (HTE).

[0016] The temperature-entropy (T-s) diagram of the thermodynamic cycle of the closed loop heat transfer system of the invention is presented in FIG. 3. The T-s diagram represents a system working with a pure or with an azeotropic working fluid. The full range on the temperature scale is 1 to 10 degrees Celsius, depending upon the frictional losses associated with the implementation of the system. Because the temperature scale is small, relative to the vapor dome, the liquidus and vaporus lines appear to be vertical. The ideal cycle, which is a Rankine cycle, is represented with dashed lines. In the ideal cycle, there are no frictional losses associated with the fluid flow, engine or the pump, therefore, some energy would be available for external work. In the real system, represented in solid lines, there are frictional losses which cause the cycle to deviate from the ideal. These irreversibilities are highlighted in grey. The work that the engine provides must be enough to overcome these irreversibilities. As represented in FIG 3, the engine operates at a high vapor quality, but not with superheated vapor. Since the system is intended for closed loop heat transfer applications, the best performance can be achieved if evaporation occurs along the entire length of the evaporator, which requires some liquid to be present. Also, since a positive displacement engine is used, lubricants can be mixed into the working fluid, and can be transported through the system as a soluble component of the liquid phase. Also, the liquid is represented as subcooling in FIG. 3. Alternatively, the liquid could be of low vapor quality.

[0017] For the system to function, the volumetric displacement rate of the turbine and turbine bypass, must be greater than the volumetric displacement rate of the pump less pump bypass. The ratio R of these two volumetric flow rates is the displacement ratio, and must be greater than unity.

• · _t — +v _s

R _ v _E +v _B _ p _y

y ^v P -Y ^v R y ^v £ ₊ ^τ y ^v S [0018] The volumetric flow rate is denoted by V , the liquid phase density is and the vapor phase density is ^ ^v . The subscripts E, B, P and R represent the engine, the engine bypass, the pump and the pump recirculation, respectively. The subscript ^ represents the liquid flow that participates in phase change (i.e. it vaporizes during the heat addition process). The subscript S represents the fluid flow that remains a single phase, whether vapor or liquid, throughout the entire process. A smaller displacement ratio tends to circulate more fluid through the system that does not change phase. The higher the fluid circulation rate, the higher the internal pressure loss in the system, which increases temperature differential from the evaporator side to the condenser side of the system. While a smaller displacement ratio increases the pressure loss, it will aid the system in operating over a wider temperature range for the same working fluid. The liquid to vapor density ratio can vary by an order of magnitude, even over a temperature ranges of 50°C. [0019] Since the displacement ratio of the engine 103 and the pump 104 is fixed, the system automatically increases the fluid flow in proportion to the heat flow, at a fixed operating temperature, when the engine 103 and pump 104 experiences no bypass or recirculation. The system pressure losses will increase with increased heat flow, therefore the pressure differential across the engine 103 and the pump 104 will also increase, leading to increases in the engine bypass and pump recirculation.

[0020] An additional consideration to the displacement ratio is the amount of condensation and vaporization occurring within the pump 104 and the engine 103. Ideally, these processes are isochoric, however, the material temperatures in the engine 103 and the pump 104 may cause some degree of condensation and evaporation, which can impact the overall displacement ratio. Additionally, if there are several evaporators in parallel, further consideration is needed for the displacement ratio of the system to prevent any individual evaporator from reaching dry-out conditions, where the fluid leaving is superheated vapor.

[0021] While nearly isothermal heat transport can be yielded by the invention with a pure working fluid, heat transfer can also be accommodated over a varying temperature input and output using a zeotropic (or non-azeotropic) mixture of materials. The thermodynamic cycle of this system on a T-s diagram is represented in FIG. 4. The evaporation and condensation processes are non-isothermal, and are referred to as temperature glide. The magnitude of the temperature glide depends on several factors, including, the mixture of fluids used, the operating pressure of the system, as well as the percent of fluid vaporized and condensed in the process. The range of the temperature scale on FIG. 4 can be 5°C to 50°C or more, depending on the mixture of materials used as well as the proportions of each material. The fluids can be a binary, ternary or other mixture. [0022] The heat input source to the invention can come from any source. Some examples of heat sources that are electronics component, a waste heat source from another process such as a steam engine, the waste heat from a condenser, the heat from a combustion process, or a solar thermal collector. The heat removed by the condenser can go into any lower temperature heat sink. Some examples of heat sinks for the condenser are cooling tower water, process water, sea water, air, or an evaporator of a vapor compression cycle.

[0023] A representation of the engine and pump assembly if presented in FIG. 5 and FIG. 6 as the assembled and blown apart views, respectively. The engine housing 205, the pump housing 206, the engine intake port 201, the engine exhaust port 202, the pump intake 203 and the pump discharge 204 are denoted. The engine has two counter-rotating lobes 207 which are mechanical coupled by several pins 211 to drive the counter-rotating gears 208 that pump the fluid. While the gears serve the functions of both pumping fluid as well as act as a timing mechanism for the lobes, to prevent lockup. The lobes and the gear are separated by a plate 210 that separates the engine and the pump. The lobes and gears have embedded bearings 209 that rotate around subsequent shafts 212. The lobe and gear design allows for reversible operation, in which the side in which heat is added to the system may switch to the side that previously removed heat and vice versa. The counter-rotation of the lobe and gears will be reversed when this exchange occurs.

[0024] Alternate implementations of the invention can have several HTEs 105 as depicted in FIG. 7 A and FIG. 7 B. In FIG. 7A, each HTE derives its power from a separate evaporator 101a, 101b, 101c and share a common condenser 102. Upstream from the condenser, a manifold can be utilized to collected the exhausted fluid from each HTE's engine. Downstream of the condenser, an accumulator 106 may be placed to collect the condensed fluid (liquid) before it is distributed to the pump of each pair. The reverse of the aforementioned system is presented in FIG. 7 B. Each HTE is coupled to a condenser 102a, 102b, 102c, while heat is supplied by a common heat source. The advantage of paring the HTE with an individual evaporator or condenser is the fluid flow to each device is automatically controlled by the heat absorbed or removed from the adjacent component.

[0025] While the present device has been disclosed according to the preferred embodiment of the invention, those of ordinary skill in the art will understand that other embodiments have also been enabled. Even though the foregoing discussion has focused on particular embodiments, it is understood that other configurations are contemplated. In particular, even though the expressions "in one embodiment" or "in another embodiment" are used herein, these phrases are meant to generally reference embodiment possibilities and are not intended to limit the invention to those particular embodiment configurations. These terms may reference the same or different embodiments, and unless indicated otherwise, are combinable into aggregate embodiments. The terms "a", "an" and "the" mean "one or more" unless expressly specified otherwise. The term "connected" means "communicatively connected" unless otherwise defined

[0026] When a single embodiment is described herein, it will be readily apparent that more than one embodiment may be used in place of a single embodiment. Similarly, where more than one embodiment is described herein, it will be readily apparent that a single embodiment may be substituted for that one device.

[0027] In light of the wide variety of heat exchange systems known in the art, the detailed embodiments are intended to be illustrative only and should not be taken as limiting the scope of the invention. Rather, what is claimed as the invention is all such modifications as may come within the spirit and scope of the following claims and equivalents thereto.

[0028] None of the description in this specification should be read as implying that any particular element, step or function is an essential element which must be included in the claim scope. The scope of the patented subject matter is defined only by the allowed claims and their equivalents. Unless explicitly recited, other aspects of the present invention as described in this specification do not limit the scope of the claims.